The generation of electricity by advanced gasification combined cycle power generation systems offers the potential for reduced power cost and lower environmental impact than standard coal-fired power plants. In these advanced systems, coal or other carbonaceous material is gasified with oxygen and the produced gas is cleaned to yield a low-sulfur fuel gas. This fuel gas is utilized in a gas turbine generation system to produce electric power with reduced environmental emissions. Because these advanced systems are more energy efficient than traditional coal-fired power plants, the amount of carbon dioxide produced for a given power output is reduced significantly. The growing interest in gasification combined cycle (GCC) technology in recent years has been stimulated by the higher efficiency and demonstrated reliability of advanced gas turbines, coal gasification processes, and air separation systems which are utilized in integrated gasification combined cycle (IGCC) systems. The proper integration of these three main components of an IGCC system is essential to achieve maximum operating efficiency and minimum power cost.
A general review of the current art in GCC and IGCC power generation systems is given by D. M. Todd in an article entitled "Clean Coal Technologies for Gas Turbines" presented at the GE Turbine State-of-the-Art Technology Seminar, July 1993, pp. 1-18. A review of various integration techniques and the impact thereof on GCC economics is given in a paper by A. D. Rao et al entitled "Integration of Texaco TQ Gasification with Elevated Pressure ASU" presented at the 13.sup.th EPRI Conference on Gasification Power Plants, San Francisco, Calif., Oct. 19-21, 1994.
The integration of air separation units and gas turbines in IGCC systems are reviewed in papers entitled "Next-Generation Integration Concepts for Air Separation Units and Gas Turbines" by A. R. Smith et al in J. Eng. For Gas Turbines and Power, Vol 119, April 1997, pp. 298-304, and "Oxygen Production in an IGCC Plant" by R. J. Allam et al in Power-Gen Europe, Cologne (Germany), May 17-19, 1994, pp. 581-596. Representative process configurations for integrated gas turbine and air separation systems are given in U.S. Pat. Nos. 5,388,395, 5,459,994, and 5,609,041 and in European Patent Publication No. EP 0 773 416 A2.
U.S. Pat. No. 5,501,078 describes a method of operating the air separation plant of an IGCC system under turndown conditions at reduced oxygen product pressure and purity.
U.S. Pat. Nos. 5,501,078, 5,224,336, 5,437,160, 5,592,834, and 5,566,825 describe process control methods for operating IGCC systems under changing oxygen and nitrogen product demand. A typical double-column air separation distillation system is used in which nitrogen-enriched liquid is withdrawn from the higher-pressure column and introduced as reflux into the top of the lower-pressure column. During periods of increasing or decreasing product demand, a portion of this nitrogen-enriched liquid is either stored to reduce the amount of reflux to the lower-pressure column or withdrawn from storage to increase the amount of reflux to the lower-pressure column. U.S. Pat. No. 5,224,336 teaches that nitrogen-enriched liquid is stored when the feed air pressure to the higher-pressure column increases, thereby decreasing reflux, and that nitrogen-enriched liquid is withdrawn when the feed air pressure to the higher-pressure column decreases, thereby increasing reflux. U.S. Pat. Nos. 5,437,160, 5,592,834, and 5,566,825 teach that nitrogen-enriched liquid is stored when a decrease in the feed air flow rate to the higher-pressure column or a decrease in product demand occurs, thereby decreasing reflux, and that nitrogen-enriched liquid is withdrawn from storage when an increase in the feed air flow rate to the higher-pressure column or an increase in product demand occurs, thereby increasing reflux.
It is well-understood in the art that control of the air separation system in response to changing oxygen product demand from the gasifier, which in turn is a result of changing electric power demand, is of critical importance for efficient IGCC system operation. Off-design or transient operation of the gas turbine can occur for other reasons, for example changes in ambient temperature, which also will impact operation of the air separation system. Since the air separation system is closely linked with both the gasifier and gas turbine systems, lack of proper control in the air separation system will have a serious negative impact on the control of the entire IGCC system.
As described in the background art cited above, the air separation system is linked with the gasifier and gas turbine of an IGCC system in several ways. First, oxygen at the proper purity, pressure, and flow rate is supplied to the gasifier to produce fuel gas for the gas turbine combustor. Second, some or all of the byproduct nitrogen at the proper purity, pressure, and flow rate is withdrawn from the lower-pressure column, compressed, and mixed with the fuel gas to the combustor to recover additional energy and reduce combustion temperatures for nitrogen oxide control. Third, some or all of the compressed air feed to the air separation system can be provided by a portion of the air from the gas turbine compressor or some other source of compressed air. In addition, high pressure nitrogen can be supplied to the gasifier for inerting and solids handling requirements.
Close control of the oxygen content in the byproduct nitrogen to the gas turbine combustor is required in order to avoid the formation of explosive mixtures when this nitrogen is mixed with fuel gas prior to or within the gas turbine combustor. This is particularly critical during periods of changing demand for oxygen product, since a change in oxygen demand results in transient operation of the entire air separation system. The present invention, as described in the specification below and defined by the claims which follow, is an improved method to operate an IGCC system during both transient and steady state operation, in particular to control the purity of byproduct nitrogen to the gas turbine combustor during transient operation.